In the Lightroom Develop module, there are different ways to look at Before and After for changes that you have made. One of my favorites is to use the backslash key \ to toggle between Before and After. The default Before state is the first step in the History panel, usually your file import. So by default, you are toggling between “before all changes” and “after all changes”.
Sometimes though, you may want to look at Before and After just the last change you made, or the last few changes. Fortunately, you have the flexibility to set which step in your processing will be the Before state: in the History panel in the Develop module, simply right-click (ctl-click for one button mac mouse users) on the step you want to assign to Before, and choose Copy History Step Settings to Before.
In the example below, my Before state will be before sharpening (which involved setting four settings). Toggling the \ key will therefore show me before and after sharpening.
Sometimes when I am using the \ key to toggle between Before and After, I will try to continue making adjustments to my image and find that Lightroom is locked up, and I can’t do anything. It is always because I have forgotten that I am still in Before mode, and changes can’t be made in this mode. If this happens to you, hit the \ key again to get back to After.